Loudspeaker cabinets



May 5, 1959 E. J. JORDAN LOUDSPEAKER CABINETS Filed Aug. 29, 1955 nvenlor By Q1/2li; A Harney;

United States Patent O LOUDSPEAKER CABINETS Edward James `lordan,Sutton, England, assignor to Goodmans Industries Limited, Wembley,England, a British company Application August 29, 19ss,seria1 No.531,002

Claims priority, application Great Britain September 3, 1954 4 Claims.(Cl. 181-31) It is well known that when a loudspeaker unit is enclosedat the rear side thereof by a cabinet of modest dimensions, theeffective speaker resonant frequency is raised so causing a reduction inor a loss of the lower frequencies. One method of obtaining a betterdevelopment of the lower frequencies is to employ the so-called reflexor vented cabinet embodying a second radiating port of approximately thesame area as the piston area of the speaker unit and facing the samedirection as said unit. Such cabinets, however, are not a completesolution to the problem. Apart from the fact that they necessitate theuse of a large cabinet which has to be designed in each case accordingto the particular speaker unit to be used, they have a resonance at ahigher frequency than the eifective speaker resonant frequency and soresult in an irregular development over the lower frequency range.

The main object of the present invention is to provide an improvedloudspeaker cabinet which gives a proper and balanced development of thelower frequencies.

More particularly, it is an object of the invention to provide aloudspeaker cabinet incorporating an improved method of loading thespeaker diaphragm which results in an overall improvement in theacoustic properties and in particular a damping and lowering of theeffective speaker resonance together with better frequency re sponse andtransient response, such advantages being achieved with a cabinet ofrelatively smaller dimensions and imposing less critical conditions asto size and form and of the initial speaker resonance.

According to the invention there is provided in or for a cabinetenclosing the rear side of a loudspeaker, an acoustic impedance unitcomprising an area of the cabinet wall or walls in which are provided aport or vent of a size sucient to produce a lowering and damping of theeffective speaker resonance but small enough to have negligibleradiation, together with a number of small openings providing aresistive relief to air compression and rarefaction.

With this construction and at low frequencies, the concentrated air massin the port or vent is added to the diaphragm mass and so in knownmanner lowers the effective resonance thereof. The frequency of thisresonance may be predetermined and can be regarded as the fundamentalresonance of the system and it is substantially independent of cabinetsize. Above this frequency, the air mass in the vent is too inert tovibrate readily and the system begins to behave as though the rear sideof the unit is enclosed completely and consequently has a secondresonance at a higher frequency, the Value of which depends upon thestiifness of the air in the enclosure and the mass and compliance of thespeaker diaphragm. However, the stiffness of the enclosed air is modiedby a ow in the multiplicity of slits or openings in the enclosure wall,and the shape and size of these openings are such that they cause animpedance to the air movement which is principally resistive(frictional) lCC and so have a considerable damping action upon theupper resonance.

Considering the equivalent electrical circuit, the slits or likeopenings may be regarded as a resistance connected in parallel with theacoustic capacitance of the enclosure and the inductance representingthe mass or inertance of the port at low frequencies, these being inseries with the radiation resistance, and it will be noted that theimproved construction results in a decrease in the impedance of theparallel section and therefore increases the power dissipated in theradiation resistance. Moreover, taking the equivalent impedance curve,it will be found that the improved construction not only results in alowering of the second peak but also in a raising of the trough betweenthe two peaks and thus by the invention there is obtained a more regulardevelopment of the low frequency range.

Two embodiments of the invention together with a modified form thereofwill now be described with reference to the accompanying drawings ofwhich:

Fig. 1 is a rear view in perspective of a loudspeaker cabinet showingone method of providing an acoustic impedance in accordance with theinvention,

Fig. 2 is a front view in perspective of a loudspeaker cabinet showing asecond method of providing an acoustic impedance in accordance with theinvention, and

Fig. 3 is a fragmentary perspective view of the acoustic impedance unitshown in Fig. 2 but modified so as to permit it to be removed orinstalled as a separate assembly.

Referring to Fig. l, the loudspeaker cabinet 1 has an aperture 2 in itsfront wall to receive the loudspeaker (not shown). In its rear wall areprovided a port or vent 3, together with a plurality of slits 4, theseopenings cooperating to provide an acoustic impedance in the manneralready described.

In carrying the invention into effect, the small additional openings 4may conveniently take the form of a series of saw cuts or equivalentslits of about one tenth of an inch Wide and be formed in one or morecabinet walls defining the speaker enclosure and not merely in the `rearwall as shown. The wall in which the slits are formed needs to be rigidand a timber panel of say one inch thickness will be found particularlysuitable for this purpose as the depth of the slit relative to its widthwill provide a large Wall area, that is, high frictional resistance toair movement, in relation to the air mass enclosed by the slit. Theslits or openings may of course be formed by other methods and in wallsof other material and need not occupy the full length of the wall asshown in Fig. l. The port or vent 3 preferably has a cross sectionalarea in the region of sixteen square inches when used with a speakerhaving an effective diameter of ten inches and may be extended inwardlyb-y a peripheral wall to increase the loading applied to the speakerdiaphragm.

The interior of the enclosure 1 may be lined with sound absorbentmaterial to absorb H.F. reections but as such material could not beapplied to the wall containing the slits without masking the slits,reflection from this wall can be avoided by suspending a curtain ofsound absorbent material between said wall and the speaker unit.

Turning to Fig. 2, the cabinet 1 is provided with aloudspeaker aperture2 as before but, this time, the acoustic impedance is provided in thefront wall of the cabinet enclosure. As already explained, it may beprovided in any wall or in more than one wall.

In this alternative method of providing the acoustic impedance there isprovided a single, larger aperture covered with a layer (or layers) ofgauze or other openwork resistive material 5, the open mesh of thelatter constituting the small openings in place of the slits 4 of Fig.1.

By leaving an open portion 6 not covered by gauze, there can be providedthe main port 3 of Fig. l. The gauze is braced by a rear grill 7 and afront grill 8, the latter being shown broken away to reveal the innerconstruction. These grills are made of wood, metal or other convenientmaterial.

In accordance with a modified embodiment of the invention, the port orvent together with the slits or equivalent openings are provided in adetachable frame unit provided with a flange or other means for enablingit to be secured in an opening in the wall of the loudspeaker cabinet orenclosure. This may be done with either of the acoustic impedance unitsalready described.

In Fig. 3 is shown a detachable unit constructed generally on the samelines as that shown in Fig. 2. This time, however, the gauze 5 providedwith a port 6 and braced by grills 7 and 8 is xed in a flanged frame 9insertable in a large aperture in the wall of the cabinet 1. The angedconstruction of the frame 9 allows it to be secured in the aperture 10by screws such as 11.

Throughout the description and claims, the general term walls isintended to include the top and bottom of the cabinet as well as theside walls.

I claim:

l. A cabinet having walls for enclosing the rear side of a loudspeaker,at least one of its walls being provided with a plurality of smallopenings providing a resistive relief to air compression and rarefactiondeveloped by the loudspeaker, and said wall being provided with anonradiating port for producing a lowering and damping of the eiectiveloudspeaker resonance.

2. A cabinet as dened in claim l, in which said small openings areprovided by at least one sheet of open-work material, and in which saidport is provided by leaving a space in the open-work material.

3. A cabinet as defined in claim 2, including a frame in which saidopen-work material is mounted to form an acoustic impedance unit, and awall of the cabinet is provided with an aperture corresponding with saidframe and in which said frame and unit are detachably mounted.

4. A cabinet having walls for enclosing the rear side of a loudspeaker,at least one of its walls having a plurality of openings suicientlysmall to provide a resistive relier" to air compression and rarefactiondeveloped by the loudspeaker and having a port communicating with theexterior of the cabinet and of a size suiiiciently large to produce alowering and damping of the effective loudspeaker resonance butsutiiciently small to have negligible radiation.

References Cited in the file of this patent UNITED STATES PATENTS1,841,101 Flanders et al Jan. 12, 1932 1,902,609 Beers et al. Mar. 21,1933 2,766,839 Baruch et al. Oct. 16, 1956 FOREIGN PATENTS 496,619 GreatBritain Dec. 2, 1938 641,718 Great Britain Aug. 16, 1950 1,065,126France Dec. 30, 1953

